Construction vehicles are known which are designed to transport large amounts of ore in mines and which roll in severe conditions, in particular on steeply inclined slopes. Such vehicles, which are referred to as dumpers, are provided with one or more driving axles and a steering axle fitted with appropriately sized tires. It has been found that the wear of such tires is markedly affected by the slippage generated by the longitudinal reaction force exerted by the ground on the tire tread as a result of the torque involved in transporting large loads.
From FR 2382348 a tire of the type fitted to an agricultural tractor is known, which has on its tread a pattern formed of blocks in relief which extend over about half the width of said tread. The irregular wear that can occur on this type of tread pattern is reduced by the application of a technique consisting in the provision of leading and trailing faces having angles measured relative to a perpendicular to the rolling surface that vary progressively from the median part of the tread towards the edges; besides, the angle of the leading face decreases towards the edge of the tread while that of the trailing face increases.
That document essentially concerns tires for vehicles working on loose ground and which have to travel on roads from time to time; unlike the tires to which the present invention relates, these tires are not stressed under load conditions which are very large and very variable during their cycle of use, and besides the service pressures are generally low on loose ground so as not to give rise to undue packing down of such ground.
In the case of the invention presented here, the conditions of use are very different because tires for vehicles used only on sites of the type described are used for carrying very heavy loads which impose very large deformations on the tires and their tread. This use involves runs under heavy load alternating with runs under no load.
Besides, in some mines there are steep inclines which make the rolling conditions even more difficult.
It is necessary to distinguish two main types of use:
type A: the vehicle rolls under full load on an ascending slope and returns along the same route; in this type of use the tires on the driving axle are subjected to large driving torques on the way up and to braking torques on the return run (in this type of use the driving forces being larger than the braking forces);
type V: conversely to type A, the vehicle rolls under full load with a braking torque on a descending slope and returns up the same slope when empty (in this type of use the braking forces are larger than the driving forces).
By analogy, rolling cycles with and without load on flat or almost flat ground resemble type-A rolling, since the driving force predominates over the braking force owing to the rolling resistance of the tires.
An objective of the present invention is to find a way of determining the fitting of tires to construction machinery which is optimum from the standpoint of wear (i.e. which results in greater durability of said tires in service).